The goal of this proposal is to understand the genetic mechanisms that orchestrate the events of gastrulation by characterizing zebrafish mutations that disrupt cell fates and cell rearrangements in the early embryo. This study will focus on two genes with key functions in mesoderm and endoderm development: cyclops (cyc) and enhancer of cyclops (eoc). The proposal has three specific aims: (i) to directly test the role of cyc and eoc in fate specification by tracing the fates of mesoendodermal progenitors that are disrupted in cyc:eoc mutants; (ii) to further elucidate the function of eoc by isolating and characterizing new mutations in eoc, including null mutations; (iii) to positionally clone the cyc gene in order to understand the molecular basis of its function in gastrulation and in development of the ventral neuroectoderm.